Chapter 14: Mendel and the Gene Idea

Heredity

• Heredity= passing of traits from parent to offspring– Reproductive cells are only way traits are passed

to offspring– Changes in somatic cells will not be passed to next

generation• Genetics= scientific study of heredity

What genetic principles account for the passing of traits from parents to offspring?

1. The “blending” hypothesis is the idea that genetic material from the two parents blends together– Kind of like blue and yellow paint blend to make green

2. The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)– Heritable factors may be rearranged, but retain identity

from one generation to the next– This hypothesis can explain the reappearance of traits

after several generations

Gregor Mendel• Gregor Mendel, Austrian monk– Documented a particulate

mechanism in the 1800s by experiments with garden pea plants

• 1860s- Studied inheritance patterns in pea plants– Character= heritable feature with multiple variations– Trait= each type of variation for a character – At this time, chromosomes and genes had not been

discovered yet• Used the term heritable factors to represent what we call a gene

Gregor Mendel

• Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

• Advantages of pea plants for genetic study– Short-life cycle– There are many varieties with distinct characters

Gregor Mendel• Parents pass characters on to offspring– Pea plants= one character is flower color

• Parents can pass on different traits– Pea plants= purple or white flower color

Gregor Mendel

• Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments

• Advantages of pea plants for genetic study– Short-life cycle– There are many varieties with distinct heritable

characters – Mating can be controlled• Each flower has sperm-producing organs (stamens) and

egg-producing organ (carpel)• Cross-pollination (fertilization between different plants)

involves dusting one plant with pollen from another

Gregor Mendel• Mendel chose to track

only those characters that occurred in two distinct alternative forms

• He also used varieties that were true-breeding in P generation• Produce offspring

of same variety when they self-pollinate

In a typical experiment, Mendel mated two different true-breeding varieties= hybridization

Gregor Mendel

• The hybrid offspring of the P generation are called the F1 generation

• When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced

Gregor Mendel

• F1 Generation: 100% purple flowers

• Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation

Gregor Mendel• Mendel reasoned that only the purple flower

factor was affecting flower color in the F1 hybrids

• The factor for white flowers was not diluted or destroyed because it reappeared in the F2

generation• Mendel called the purple flower color a

dominant trait and the white flower color a recessive trait

Pea Plant Experiments•7 Characters in Pea Plants•Crossed different varieties of each character

• Hybrids•Genetic Test Cross

• P generation• F1 generation• F2 generation

•Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring

Mendel’s Hypotheses Based on 4 Concepts

1. There are alternate forms of genes (alleles) and these account for the variation of inherited characters.

2. Offspring inherit 2 alleles for each character, one from each parent.

3. When alleles are different, one allele determines appearance (dominant allele) and other has no effect (recessive allele).

4. Mendel’s Law of Segregation: Allele pairs separate during gamete production and sex cells carry one allele for each inherited character.

1. Alternate forms of genes (alleles) account for variation

• Alleles= alternative forms of genes• Flower Color: 2 Forms

2. Offspring inherit 2 alleles for each character, one from each parent.

• 1 chromosome with alleles for genes are inherited from mom and one from dad

• Alleles can be the same or different– Homozygous- alleles

same– Heterozygous- alleles

different

3. When alleles are different, dominant allele masks recessive allele

• Complete Dominance• Dominant Alleles (uppercase letters)– Determines appearance of offspring if present• Homozygous Dominant= PP

• Recessive Alleles (lowercase letters)– Masked by dominant allele– Only determines appearance if both alleles are

recessive – Homozygous recessive= pp

• Heterozygous: 1 dominant, 1 recessive= Pp

Frequency of Dominant Alleles

• Dominant alleles are not necessarily more common in populations than recessive alleles– For example, one baby out of 400 in the United States

is born with extra fingers or toes• The allele for this unusual trait is dominant to the

allele for the more common trait of five digits per appendage

• In this example, the recessive allele is far more prevalent in the population than the dominant allele

Exceptions to the Rule• Incomplete Dominance:

Expression of recessive allele not completely masked in heterozygote– Snapdragon flower color

• Co-dominance: Two alleles are dominant and expressed in heterozygote– Example: Blood Type: AB

Red = RR

White = rr

Pink = Rr

4. Mendel’s Law of Segregation

• Allele pairs separate during gamete production and sex cells carry one allele for each inherited character

• Meiosis- haploid number of chromosomes• Fertilization- fusion of egg and sperm result in

diploid offspring– Genotype= alleles present in offspring for character

• Example: PP, pp, Pp

– Phenotype= physical appearance as a result of alleles• Purple flowers, white flowers

Homologous chromosomes can have different alleles

G1

S phase

4. Mendel’s Law of Segregation

• Mendel derived the law of segregation by following a single character

• The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character

• A cross between such heterozygotes is called a monohybrid cross

Mendel’s Experimental Crosses

P Generation

PP pp

Cross homozygous dominant plant with homozygous recessive plant

Mendel’s Law of Segregation

Genotype and Phenotype

• Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition– Phenotype= physical appearance– Genotype= genetic makeup

• Flower color in pea plants:– PP and Pp plants have the same phenotype

(purple) but different genotypes

Heterozygotes

• Organisms with heterozygous genotype are considered “carriers” of the recessive allele– Not a true-breeding variety– Pea Plants Flower Color= Pp

• Recessive allele is present, but not “visible” in phenotype

• Can be passed on to next generation

Punnett Square• Used to predict potential traits in offspring– 4 possible combinations of alleles– Each parent creates 2 potential gametes 2 x 2= 4

• Need to know type of gametes produced by parent– Genotype= alleles for character– Each gamete has one allele

AA Aa

AA Aa

Pea Plants Experiment •Cross F1 generation to produce F2 generation•Phenotype: Purple Flowers•Genotype: Heterozygous•Cross Bb X Bb

•F2 generation: • 1 BB (purple)• 2 Bb (purple)• 1 bb (white)

Research in Genetics

• Testcross– Mating between unknown

genotype and homozygous recessive genotype

– Use Punnett Squares to predict offspring traits and compare to actual outcomes

Mendel’s Law of Independent Assortment

• Mendel identified his second law of inheritance by following two characters at the same time

• Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters

• A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

Mendel’s Law of Independent Assortment

• Mendel’s Law of Independent Assortment– Genes inherited independently of each other– Inheritance of one character independent from

another character • This law applies only to genes on different

chromosomes or those far apart on the same chromosome– Genes located near each other on the same

chromosome tend to be inherited together

Mendel’s Law of Independent Assortment

Genetic Inheritance

• Mendel’s laws of segregation and independent assortment reflect the rules of probability

• When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss– In the same way, the alleles of one gene segregate

into gametes independently of another gene’s alleles

Genetic Inheritance

• The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities

• Probability in an F1 monohybrid cross can be determined using the multiplication rule

• Segregation in a heterozygous plant is like flipping a coin: – Each gamete has a ½ chance of carrying the dominant

allele and a ½ chance of carrying the recessive allele

Genetic InheritanceThe addition rule=

probability that any one of two (or more) exclusive events will occur is calculated by adding together their individual probabilities

Probability of heterozygote: ¼ + ¼ = ½

Probability of homozygote:¼ + ¼ = ½

Dihybrid Crosses• A dihybrid or other multicharacter cross is

equivalent to two or more independent monohybrid crosses occurring simultaneously

• Crosses with 2 characters of interest– RrYy x RrYy• In gametes, alleles for one character can be paired with

either allele for other character• For each parents gametes, R can be paired with Y or y • 4 possible gamete types for each parent

–RY, Ry, rY, ry

Dihybrid Cross

Exceptions to the Rule

• The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied

• However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

Exceptions to the Rule

• Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations:

1. When alleles are not completely dominant or recessive

2. When a gene has more than two alleles3. When a gene produces multiple phenotypes

Exceptions to the Rule• Incomplete Dominance:

Expression of recessive allele not completely masked in heterozygote– Snapdragon flower color

Red = RR

White = rr

Pink = Rr

Exceptions to the Rule• Co-dominance: Two alleles are dominant and expressed

in heterozygote• This occurs in blood types, which also has more than 2

alleles– Four phenotypes: A, B, O, AB– A and B codominant, both are dominant to O

• Determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i

• The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate

• Enzyme encoded by the i allele adds neither

Pleiotropy

• Most genes have multiple phenotypic effects, a property called pleiotropy

• For example, pleiotropic alleles are responsible for the multiple symptoms of some hereditary diseases– Cystic fibrosis– Sickle-cell disease

Sickle-cell disease

• The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells– Causes cell to collapse and form sickle shape– Clogs blood vessels

• In homozygous individuals, all hemoglobin is abnormal (sickle-cell)

• Symptoms include physical weakness, pain, organ damage, and even paralysis

Sickle-Cell Disease• Heterozygotes= produce

both types of blood cells– Heterozygotes have sickle-cell

trait• Usually healthy but may suffer

some symptoms of disease

• Sickle-cell disease affects one out of 400 African-Americans – Unusual to have high

frequency of an allele with detrimental effects in homozygotes

Normal Blood Cell

Sickle-Cell

Sickle-Cell Disease• Reason: “Heterozygote

Advantage”– Less susceptible to the

malaria parasite, so there is an advantage to being heterozygous in areas where malaria is common

– Keeps allele in population despite detrimental effects of homozygous condition

Normal Blood Cell

Sickle-Cell

Epistasis• In epistasis, a gene at one locus alters the

phenotypic expression of a gene at a second locus

• In Labrador retrievers and many other mammals, coat color depends on two genes– One gene determines the pigment color (with

alleles for black or brown)– The other gene (with alleles

for color or no color) determines whether the pigment will be deposited in the hair

Polygenic Inheritance• Quantitative characters are those that vary in

the population along a continuum• Quantitative variation usually indicates

polygenic inheritance, an additive effect of two or more genes on a single phenotype

• Skin color in humans is an example of polygenic inheritance

Environmental Variation in Phenotypes

• Another exception arises when phenotype of a character depends on environment as well as genotype– Multifactorial characters: genetic and

environmental factors collectively influence phenotype

• Norm of reaction: phenotypic range of a genotype influenced by the environment– Usually broadest for polygenic characters

• Hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity

Human Genetics

• Humans are not good subjects for genetic research – Generation time is too long– Parents produce relatively few offspring– Breeding experiments are unacceptable

• However, basic Mendelian genetics still serves as the foundation of human genetics

Genetic Traits in HumansPedigree= Family tree of genetic traits for multiple generations

Pedigrees• Pedigrees can also be used to make predictions about

future offspring• We can use the multiplication and addition rules to

predict the probability of specific phenotypes• Many genetic disorders are inherited in a recessive

manner– Recessively inherited disorders show up only in individuals

homozygous for the allele• Carriers are heterozygous individuals with 1 copy of recessive allele

but are phenotypically normal• Most individuals with recessive disorders are born to carrier parents

• Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

Albinism in Humans

Human Genetics• For a growing number of diseases, tests are available

that identify carriers and help define the odds of offspring having disease more accurately– Many diseases, such as heart disease, diabetes, alcoholism,

mental illnesses, and cancer have both genetic and environmental components, making predictions for multifactorial diseases difficult

• Now that the human genome has been described, research is focusing on identifying mutations in sequence and how they affect health– Advances in understanding the human genome will

potentially lead to new medicines, treatments, and preventive care

Current Genetic Research

• Breast Cancer– BRCA1 and BRCA2- human genes part of a group

of genes called tumor suppressors• Stabilize DNA and prevent uncontrolled growth

– Mutations in these genes increase risk of hereditary breast and ovarian cancers

– Mutations can be found in men and women and may increase risk of other types of cancers

– Blood test can detect genetic mutation• Not all mutations are harmful, many types are neutral

Current Genetic Research

• Using family histories, genetic counselors help couples determine the odds that their children will have genetic disorders– Individuals can evaluate chances of development

of genetic disorders present in close family members

– Potential parents with genetic disorders can determine the chances of passing genetic disorder on to offspring

Current Genetic Research• Punnett Square: Predicting the chance of

passing genetic disorder on to offspring– If a woman has a blood test and finds she has a

recessive mutation for a genetic disorder (heterozygous), what are the chances of her offspring having the mutation if her husband is homozygous dominant?

AA Aa

AA Aa

A a

A

A

Genotype Probability:50% AA (no mutation)50% Aa (mutation carrier)

Inbreeding

• In a large population, chance of mating with a relative is low because there are many choices– Most societies and cultures have restrictions against

marriages between close relatives• Sexual reproduction between non-related individuals

increases or maintains high genetic diversity• In a small population, chance of mating with a

relative is high because there are few choices– Increases frequency of deleterious recessive genotypes– Exposes rare deleterious mutations

Example: White Tigers• White coloration in Bengal

tigers– 1 in 15,000 wild births

• Homozygous recessive condition (aa)

• In 1951, one white male tiger was captured in the wild

• Most white tigers in zoos are descendants of this original tiger

Practice • Use a Punnett Square to find probabilities of

offspring from the following matings:1. White male (aa) x Orange female (AA)2. White male (aa) x Orange daughter (Aa)3. White male (aa) x White daughter (aa)

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Example: White Tigers

• Over time, repeated matings between close relatives to produce white tiger decreased genetic diversity and evidence of inbreeding depression was observed– Birth defects and low reproductive success– Weak immune system- more susceptible to disease– Poor eyesight and hearing– Weak bones

• In 2011, American Zoological Associationbanned breeding of white tigers